The present invention relates to the field of nanotechnology and, more particularly, to the field of nanocomposites.
Inorganic nanocomposites have recently emerged as a means of controlling material functionality through morphology, as well as composition, to give rise to combinations of properties not generally found in homogeneous single-phase materials. For example, battery electrodes must efficiently conduct both electrons and ions to achieve high power, while, conversely, thermoelectric energy conversion is most efficient when electrical conductivity is high, yet thermal conductivity is low. However, the development of nanocomposites for such applications is hindered by the lack of a general fabrication method capable of controlling morphology over a wide range of compositions. Recently, exquisite control over colloidal nanocrystal assembly has been developed, including highly ordered superlattices (Pileni, J. Phys. Chem. B 105, 3358-3371 (2001)), binary nanocrystal assemblies (Shevchenko et al., Nature 439, 55-59 (2006); and Smith et al., J. Am. Chem. Soc. 131, 3281-3290 (2009)), and oriented nanorod assemblies (Cozzoli et al., Chem. Soc. Rev, 35, 1195-1208 (2006); Ryan et al., Nano Lett. 6, 1479-1482 (2006); Gupta et al., Nano Let. 2066-2069 (2006); and He, J. et al. Small 3, 1214-1217 (2007)).
These methods for assembling organic-ligand terminated nanocrystals have been refined over the last decade or so. For example, vertically oriented arrays of semiconductor nanorods have been prepared by controlled evaporation, electric field orientation, or evaporation at the air-water interface, all of which depend on the organic ligand coating to mediate nanorod interactions with each other and their environment. Similarly, control over binary nanocrystal assemblies was found to depend critically on the presence of ligands (Shevehenko et al.). Recently, electronic applications of nanocrystal assemblies have gained momentum since it was demonstrated that their conductivity can be greatly enhanced by post-assembly replacement of bulky organic ligands with much smaller molecules, such as hydrazine (Talapin et al., Science 310, 86-89 (2005)) or ethylenediamine (Murphy et al., J. Phys. Chem. B 110, 25455-25461 (2006)). This process takes full advantage of nanocrystal assembly techniques so that, for example, heterogeneous doping could be observed in binary assemblies of Ag2Te and PbTe nanocrystals (Kang et al., Nature 458, 190-193 (2009)). However, the electronic properties of such materials are strongly history-dependent due to the reactivity and volatility of the small molecules employed (Talapin et al.).
Compelled by the unique properties achievable in inorganic nanocomposites, several approaches to their fabrication have been demonstrated for specific applications. For example, spinodal decomposition and precipitation of a secondary phase of PbS within a PbTe matrix was used to generate a nanostructured thermoelectric composite (Androulakis et al., J. Am. Chem. Soc. 129, 9780-9788 (2007)). While remarkable improvements in thermoelectric efficiency resulted, the ability to tune morphological characteristics such as the size of the nano-inclusions is limited and the achievable compositions are severely restricted. A more general approach, which has been applied to battery electrodes, is to mechanically mill the component materials until they intermix on the nanoscale (Badway et al., J. Electrochem. Soc. 150, A1209-A1218 (2003)). The cost of generality, however, is a failure to reliably create intimate contact between the components, and morphology is again poorly controlled.
Another elegant example is the co-assembly of solution-processed building blocks into ordered arrays of gold nanoparticles within a silica matrix, which provides thermal stability (Fan et al, Science 304, 567-571 (2004)). However, this method cannot be applied to arbitrary compositions since it relies on the carefully balanced interaction of the two components, together with a structure-directing surfactant, under dynamic solvent evaporation conditions.
Finally, in a recent report, solution-phase ligand exchange in a strongly reducing environment was used to adsorb CMCs to the surfaces of dispersed nanocrystals (Kovalenko et al., Science 324, 1417-1420 (2009)). These could then be deposited to form composite films, although the harsh chemical environment limits the compositional applicability and general approaches to assemble charged nanocrystals are lacking Underscoring these limitations, in only one case gold nanoparticles with Sn—S ligands—was preparation of an ordered assembly demonstrated and no evidence for ordering on a large scale was reported.